Advances in understanding biological systems have relied on applications of fluorescence microscopy, flow cytometry, versatile biological assays, and biosensors. These experimental approaches make extensive use of organic dye molecules as probes. But intrinsic limitations of these conventional dyes, such as low absorptivity and poor photostability, have posed great difficulties in further developments of high-sensitivity imaging techniques and high-throughout assays. As a result, there has been considerable interest in developing brighter and more photostable fluorescent nanoparticles. For example, inorganic semiconducting quantum dots (Qdots) are under active development and now commercially available from Life Technologies (Invitrogen). (Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P, Science 1998, 281, 2013-2016. Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538-544.) An alternative fluorescent nanoparticle is dye doped latex spheres, which exhibit improved brightness and photostability as compared to single fluorescent molecules because of multiple dye molecules per particle and the protective latex matrix. (Wang, L.; Wang, K. M.; Santra, S.; Zhao, X. J.; Hilliard, L. R.; Smith, J. E.; Wu, J. R.; Tan, W. H. Anal. Chem. 2006, 78, 646-654).
Fluorescence semiconducting polymer dots exhibit significant improvement in fluorescence brightness and photostability as compared to Qdots and dye-loaded latex beads. (Wu, C.; Szymanski, C.; Cain, Z.; McNeill, J. J. Am. Chem. Soc. 2007, 129, 12904-12905. Wu, C.; Bull B.; Szymanski, C.; Christensen, K.; McNeill, J. ACS Nano 2008, 2, 2415-2423.) The fluorescent polymer dots possess arguably the highest fluorescence brightness/volume ratios of any nanoparticle to date, owing to a number of favorable characteristics of semiconducting polymer molecules, including their high absorption cross sections, high radiative rates, high effective chromophore density, and minimal levels of aggregation-induced fluorescence quenching, resulting in fluorescence quantum yields that can be in excess of 70%, even for pure solid films. The use of fluorescent polymer dots as fluorescent probes also confers other useful advantages, such as the lack of heavy metal ions that could leach out into solution and which would be toxic for living organisms or biological cells.
However, a great bottleneck in nanoparticle development is the controlled chemical functionalization. There are two sets of challenges here. The first challenge is simply the design and introduction of functional groups into the side chain and backbone of the conjugated or semiconducting polymer without adversely affecting the collapse (as well as the stability and performance) of the polymer into a nanoparticle form while orienting the functional groups on the particle surface for bioconjugation, which is needed for most applications towards cellular labeling.
Previous attempts at introducing functional groups onto chromophoric polymer dots involved functionalizing the side chains of the polymer with hydrophilic functional groups at a high density (e.g. half of the monomeric units of the polymer has at least one side chain functionalized with a functional group). Although it has been claimed that the chromophoric polymers functionalized in this fashion can be formed into nanoparticles, the resulting nanoparticles tend to aggregate and degrade over time, and thus are in fact not stable (Moon et al. Angewandte Chemie. 2007, 46, 8223-8225). In fact, the heavily functionalized chromophoric polymers are more like conjugated polyelectrolytes because of the hydrophilic nature of the functional groups or side chains, and these nanoparticles are actually loose aggregates of polymers that are more like polyelectrolyte molecules (Moon et al. Chem. Communications 2011, 47, 8370-8372). The loose aggregates are formed without involving too much of polymer chain folding, and their loose structure is different from the compact chromophoric polymer dots collapsed from hydrophobic polymers. Correspondingly, these nanoparticles are unstable and offer poor performance for fluorescence labeling, and their aggregation behaviors are affected by polymer concentration, ionic strength, and temperature (Moon et al. Chem. Communications 2011, 47, 8370-8372). Furthermore, the nanoparticle formation of the heavily functionalized chromophoric polymers can require the use of harsh conditions (e.g. high concentration of acids), and this fact is supported by the common accepted understanding of polyelectrolyte conformation: the charges on a polyelectrolyte chain will repel each other (caused by Coulomb repulsion and/or solvation of the hydrophilic moieties), which causes the chain to adopt a more expanded, rigid-rod-like conformation. If the solution contains a great deal of added salt or acid, the charges will be screened and consequently the polyelectrolyte chains will associate with each other to form loose aggregates. As a result, chromophoric polymer dots generated from polymers that are more like conjugated polyelectrolytes generally are difficult to form (require harsh conditions such as acids), and once formed, the loose aggregates are unstable over time and offer poor performance, and their aggregation nature are affected by many factors such as polymer concentration, ionic strength, and temperature (Moon et al. Chem. Communications 2011, 47, 8370-8372), which limits the shelf life of these particles and degrades their performance towards biological applications. Polymers that are more like conjugated polyelectrolytes also have poor solubility in an organic solvent, and thus, they can be difficult to be made into nanoparticles using the precipitation method. With regard to the collapse and stability and performance of the chromophoric polymers, there are two additional considerations besides simply the density of the hydrophilic functional groups mentioned above. The first consideration is that the presence of a high density of either hydrophilic functional groups or hydrophilic side chains or hydrophilic moieties will adversely affect the collapse and stability of the formed nanoparticles. The second consideration is more subtle and it deals with the distribution of hydrophilic functional groups. Here, for the same number of hydrophilic functional groups, when it comes to the collapse and stability and performance of the chromophoric polymer dots, it would be much better to have these functional groups concentrated and localized to a small number of monomers rather than have the functional groups distributed homogeneously or more evenly among the monomers. This invention teaches the importance of these design considerations in forming stable and compact chromophoric polymer dots.
The other challenge for controlled chemical functionalization is to develop chromophoric polymer dot with pre-defined number of functional groups. Due to the presence of multiple reactive sites on a nanoparticle surface, it is extremely difficult to control the number and geometrical distribution of chemical functional groups. Nanoparticle multivalance can cause cross-linking of surface proteins that may activate signal pathways and dramatically reduce receptor binding capability. (Howarth, M.; Liu, W.; Puthenveetill, S.; Zheng, Y.; Marshall, L.; Schmidt, M.; Wittrup, K.; Bawendi, M.; Ting, A. Nat. Methods, 2008, 5, 397.) Therefore, there remains a need to develop fluorescent polymer dots with pre-defined (e.g. mono or bi valent) number of functional groups that allow for further conjugation to biomolecule in a defined (e.g. one-to-one) stoichiometry. Chromophoric polymer dots with pre-defined number (e.g. mono and bi valent) of functional groups bring forward unique properties of highly fluorescent nanoparticle bioconjugates for a wide range of fluorescence-based applications.